1. Field of the Invention
The present invention relates to a level converting circuit for converting an amplitude of a signal, and particularly to a level converting circuit using an insulated gate field effect transistor of a single conductivity type.
2. Description of the Background Art
In conventional semiconductor devices, there has been widely employed a CMOS circuit formed of a P-channel MOS transistor (insulated gate field effect transistor) and a N-channel MOS transistor. Based on characteristics of threshold voltages of the MOS transistors, it is general in the CMOS circuit to turn on the P-channel MOS transistor when a signal at an H level (logically high level) is to be output, and to turn on the N-channel MOS transistor when a signal at an L level (logically low level) is to be output. In the CMOS circuit, charging/discharging current flows when an output signal of the CMOS circuit changes, but no current flows when the output signal is stable, so that power consumption can be made small.
There are some cases that an internal voltage at a level different from a power supply voltage and a ground voltage is used in semiconductor devices. When the internal voltage is higher than the power supply voltage or is lower than the ground voltage, a signal changing between the power supply voltage and the ground voltage must be converted to a signal changing between the internal voltage and the ground voltage, between the power supply voltage and the internal voltage, or between first and second internal voltages, and a level converting circuit is required for such conversion.
When the level converting circuit is formed of a CMOS circuit, P- and N-channel MOS transistors must be used, resulting in an increased number of manufacturing steps. In order to avoid such increase, a level converting circuit may be formed of a single kind of MOS transistors, as is disclosed in Japanese Patent Laying-Open No. 2002-328643, for example.
The level converting circuit disclosed in the above prior art reference converts a signal changing between a ground voltage and a power supply voltage to a signal changing between the ground voltage and an internal voltage VDD2 higher than power supply voltage VDD1. The level converting circuit disclosed in the above prior art reference includes an input stage formed of an N-channel MOS transistor connected in series to a diode-connected load element and having a gate receiving an input signal, a push-pull output stage formed of N-channel MOS transistors connected in series between an internal voltage supply node and a ground node, and a capacitance element connected between an output node of the push-pull output stage and an output node of the input stage. A MOS drive transistor on a high side of the output stage has a gate coupled to the output node of the input stage, and an input signal is supplied to a gate of a MOS drive transistor on a low side of the output stage.
The capacitance element is utilized as a bootstrapping capacitance. It is now assumed that the input signal is at the low level, the drive transistor in the input stage is off, and the drive transistor on the low side in the output signal is off. In this state, when the voltage level of the output signal applied from the output stage rises in accordance with the input signal, a bootstrapping effect of the capacitance element raises the gate voltage of the high-side MOS drive transistor in the output stage to a level higher than internal voltage VDD2, to produce a signal at a level of the voltage VDD2.
When the input signal is at the high level, the low-side MOS transistor in the output stage drives the output signal to the ground voltage level. In this operation, the output signal of the input stage attains a low level of a voltage level determined by on-resistances of the diode-connected load MOS transistor and the drive transistor, and the high-side MOS drive transistor in the output stage turns non-conductive.
In the above prior art reference, only N-channel MOS transistors are used in the level converting circuit for the purpose of eliminating steps of forming P-channel MOS transistor, to reduce the number of manufacturing steps.
In the structure of the level converting circuit disclosed in the above prior art reference, the high-side MOS drive transistor in the output stage has the gate set to the electrically floating state, and through the bootstrapping operation of the capacitance element, the voltage level of the gate raised to produce a signal at the high level of voltage VDD2 higher than the high-level voltage VDD1 of the input signal. Both low-level voltages of the input signal and the output signal are equal to the ground voltage. The drive transistor in the input stage and the low-side MOS driver transistor in the output stage are commonly supplied with the input-signal, so that the level conversion of the input signal to the high-level voltage can be performed.
However, when N-channel MOS transistor is used, the low level of the output signal cannot be made lower than the ground voltage. If the low-side MOS transistor in the output stage is coupled to a negative voltage supply instead of the ground node, the low-side MOS drive transistor in the output stage does not turn non-conductive even when its gate attains the level of ground voltage. Consequently, a through-current flows in the output stage, and the voltage of output signal at the high level lowers.
If a signal at a low level of a negative voltage lower than the ground voltage is to be produced, in the structure of the foregoing prior art reference, voltage polarities are made inverted, and the MOS transistors are formed of P-channel transistors. In this case, however, the high-level voltages of the input and output signals are both equal to the power supply voltage.
In the structure of the foregoing prior art reference, therefore, the low-level voltage of the input signal could not be converted to a voltage lower than the low-level voltage with only the N-channel MOS transistors. Likewise, it is impossible to produce an output signal having a high-level voltage higher than the high-level voltage of the input signal with only P-channel MOS transistors.
In addition, in the structure of the foregoing prior art reference, both the high- and low-level voltages of the input signal could not be converted with a common circuit structure.